Film sound recorder



Nov. Z4,- v1942. G. l, DIMMICK FILM souwn RECORDER- Filed sept. 28,1940

' 2 Sheets-Sheet 1 www@ Snventr Gttomeg Nov. 24,' 1942.. G. L. DIMMICK l 2,303,199

I FILM SOUND RECORDER v `Filed Sep-t. 28., 1940 j 'l 2 Sheets-Sheet 2 :inventor Patented Nov. 24, 19142 A FILMSOUND RECORDER Glenn L. Dimmick, Haddoniield, N. J., assignor to Radio Corporation of America., a corporation of Delaware Application september 2s, 19in, serial No. 358,313

This invention relates to sound recording and, more particularly, to ground noise reduction amplifiers used in such recording.

In some of the ground noise reduction ampliyfiers previously used, the shutterv opening or in.-

crease in average brightness of light varied practically linearly with the amplitude of the impressed signal. 4In a modifiedl form of amplier using exponential tubes, the response of the amplilier varies exponentially over a portion of its range and linearly over the'balance of the range with increase in signal.

In order to keep the ground noise at a minimum when there is no signal, the opening of the shutter should be kept to a minimum, or, if a.

vvariation in light intensity is used, the light should be kept at a minimum when there is no signal. When signal is applied, however, the output of the amplifier should rise rapidly to provide sufficient clearance so that there will be no clipping of the signal peaks and, after this clearance is obtained, the output of the amplifier should rise approximately linearly with increase in signal. This result is accomplished in the present invention by loading the input transformer of the rectifying amplifier with a resistor'in series with a i'ectier which may be of the copper oxide type or of any other equivalent type which has a very` high resistance up to a fixed voltage after which the resistance drops. In the present apparatus, a copper oxide rectifier is used which has the eiect of open-circuiting the transformer secondary at low signal inputs, thereby giving again of approximately 18 db. in the amplifier output at lowsignals and a linear gain ofthe usual amplirude at higher signals.

One object of the invention is to provide an improved ground noise reduction amplier.

Another object of the invention is to provide a ground noise amplifier which will give a very high gain at low signal amplitudes.

Another object is to providea ground noise reduction amplifier which will give a high ygain to a predetermined signal amplitude and a low gain above that amplitude.

Anotherl object of the invention is to provide a` ground noise reduction amplifier which will permit using a minimum clearance of a ground noise reduction shutter at low amplitudes and which will provide adequate clearance when a signal is applied.

'Cil

following speccation and an inspection of the accompanying drawingsin which Figure 1 is a schematic diagram of the rectifying portion of an amplifier constructed in accordance with this invention,

Figure 2 is a graph showing the characteristics of the usual ground noise reduction amplifier indicating the width of the bias line as plotted against A.-C. amplitude,

Figure 3 is a graph showing the characteristics of the exponential type of amplifier wherein the width of the bias line increases exponentially with the signal amplitude to a certain point where it kbecomes linear,

Figure 4 is a graph showing the shutter opening plotted against; signal amplitude with the amplifier of the present invention,

Figure 5 shows the volt-ampere characteristic of a copper oxide rectifier composed of twelve discs in series,

Figure 6 shows the relation between shutter current and percent modulation in the case of an amplifier using the` copper oxide rectifier the characteristics of which are given in Figure 5, and

Figure 7 is a schematic diagram of a commercial typeof ground noise reduction amplifier incorporating the features of this invention.

Referring first to Fig. 1, the audio frequency current is fed to this rectifying amplifier through the transformer I0. The output of the transformer I0 passes through the rectifier I2 and is ltered by the resistance capacity filter including the capacitors I3 and I6 and the resistors I4 and I5. The rectified and ltered output is impressed on the control grid of the tube Il, the plate current of which operates the ground noise reduction shutter or other corresponding device I 9. The tube I1 is controllable by the variable l grid resistor I8. The secondary Il of the transrectifier 2|, the resistor 20 and the battery 22 all Other and incidental objects will be apparent to those skilled in the art from a reading 0f the 55 connected in series. The resistor 20 in this case is reduced to a value of 1400 ohms. If desired, the batteryzz may be omitted and sufficient additional rectifer units added in the rectiierll to provide the desired voltage characteristics. For

. example, one copper oxide rectifier unit will not pass current until a voltage of approximately .1 volt; appliedand, inthe opposite direction,

This resistor it requires. of course, a considerably greater voltage. If, therefore, I desire to have the secondary Il of the transformer i act as if it were open-circuited at any voltage below 1.2 volts, a single cell may be used at 22 having a voltage of approximately 1.1 and a rectifier of appropriate character may be used at 2 I It is undesirable to use dry cells in recording equipment due to their variation in characteristics and, accordingly, for commercial use I provide a rectifier consisting of 12 copper oxide units as indicated at 4I in Fig. '7.

The circuit shown in Fig. 7 is that of a commercial 'ground noise reduction amplifier with my invention applied thereto, and the portions thereof which correspond with the elements shown in Fig. 1 are appropriately indicated. This amplifier is known as the RCA-M1-3218-C. The input transformer i0 of Fig. 1 corresponds to the transformer 30 in Fig. 7. The secondary 3i of .the transformer is shunted by the rectifier Il and resistor 40 in series.

The rectifier 4l is composed of twelve copper oxide elements which give the equivalent of approximately 1.2 volts internal bias. In other words, the effective resistance of the rectifier 4| is about 10,000 ohms for voltages below 1.2

-volts and drops to about 250 ohms for higher voltages, giving a total effective resistance of about 1650 ohms with the series resistor 40. 'I'he current rectified by the rectifier 32 is filtered by the resistance capacity filter including the capacitors 33 and 36 and the resistors 34 and 35, and the output is fed to the tube 31.

This arrangement gives an effective gain of approximately 18 db. in the voltage produced in the main rectifier-circuit up to 1.2 volts, the `effect produced-being indicated by the graph in Fig. 4 where, at low signal levels, the shutter is opened rapidly while, over the usual useful volume range for reproduction, the shutter opening varies linearly with amplitude. It will be apparent that on most signals, which only reach their maximum amplitude after several cycles, the effect of this arrangement is substantially the same as if anticipation'were used or as if a delay circuit were inserted in the signal channel andthe ground noise reduction amplifier actuated from a signal take-off preceding the delay circuit.

The volt-ampere characteristics of the copper oxide rectifier such as used in Fig. 'l are illustrated in Fig. 5 and the resulting shutter current in milliamperes, as plotted against percent modulation, are shown in Fig. 6.

It will be apparent to those skilledin the art that the invention is not limited to the use of a copper oxide rectifier but that such a rectifier is described and is adapted for use commercially due to its small size and the fact that it requires no auxiliary source of either potential or current. It is within the scope of the invention to use any other type of rectifier adapted by its characteristics to the requirements of the particular circuit under consideration. For example, a thermionic diode rectifier may be used and will in some respects give better operating characteristics, although it is less convenient to A incorporate into a compact or portable circuit. Such a rectifier, for example, has practically zero starting voltage and, therefore, requires the use of a source of opposing potential in series there with, as shown in Fig. 1. On the' otherhand, this type of rectifier may be chosen so as to have Apractically any desired internal resistance, thereby avoiding the necessity for the series resistor tremely low internal resistance.

20. Conversely, if a rectifier such as a mercury vapor tube is used, a starting voltage of approximately 15 is secured, but the rectifier has an ex- The use of this rectier would accordingly avoid the use of the series source of potential 22 for a voltage of I5 but would require the use of a series resistor, such as 20, having substantially the entire resistance desired for loading the transformer. Many other types of rectifiers are known in the art, such, for example, as the various types of crystal detectors and electrolytic detectors. The particular type of rectifier used and whether or not an opposing potential or series resistor are used should be determined in accordance with the requirements of the particular circuit, the characteristics of the various rectifiers being well known in the art.

I claim as my invention:

1. A ground noise reduction amplifier including an input transformer, a rectifier of predetermined starting voltage in parallel with the secondary of said transformenand a second rectifier in series with the output circuit of the combination of said secondary 4and said first-mentioned rectifier.

2. A ground noise reduction amplifier including an input transformer, a rectier operating 4in a certain direction in parallel with the secondary of said transformer, a source of potential in series with said rectifier, and a second rectifier operating in the same direction as said rstmentioned rectifier in series with the. output circuit of the combination of said secondary and said first-mentioned rectifier. I

3. A ground noise reduction amplifier including a transformer, a rectier of predetermined starting voltage and internal resistance in parallel with the secondary of said transformer, and a second rectifier in series with the output circuit of the combination of said secondary and said first mentioned rectifier.

4. A ground noise reduction amplifier including an input transformer, a rectifier operating in a certain direction of predetermined starting voltage in parallel with the secondary of said transformer, a loading resistor in series with said rectifier across said secondary, and a second rectier operating in the same direction as said first-mentioned rectifier and connected in series with the output circuit of the combination of said loading resistor, said secondary and said firstmentioned rectifier.

5. A ground noise reduction amplifier including an input transformer, a copper oxide rectifier of predetermined starting voltage in parallel with the secondary of said transformer, a loading resistor in series with said rectifier across s aid secondary, and a rectifier in series with the output circuit of the combination of said loading resistor, said secondary and said first-mentioned rectifier.

6. A ground noise reduction amplierincluding a transformer, a rectifier of predetermined starting voltage in parallel with the secondary of said transformer, a loading resistor in series with said rectier across said secondary, a second rectier in series with the output circuit of the combination of said loading resistor, said secondary and said first-mentioned rectifier, and a filter circuit connected to the output of said rectifier.

'1. A ground noise reduction amplifier including a transformer, a rectifier of predetermined starting' voltage in parallel with the secondary of nid transformer. a loading resistor in series la,sos ,1o9-- a Y v with said rectifier across saidl` secondary. a rectiiier in series with the output circuit of the combination of said loading' resistor, said secondary and 'said first-mentioned rectiiier, a'. filter cir-1l cuit connected to the output of said rectifler, and

'a D. C. 'ampliiier connected to thev output of saidvfilter circuit.

8. A'ground noise reduction ampliiier including`a transformer, a rectiiier operating in a certain direction of predetermined starting voltage in parallel `with the secondary of said transformer, a loading resistor tiiier across ,said secondary,a rectifier operating in the same direction as said mst-mentioned*- rectiiierr and connected in series with the output 'circuit .or the combination or said loading rein series with said recsistor, said secondary 'and said first-mentioned rectifier, a filter circuit connected to the loutput of said rectifier, a D. C. amplifier connected to the output oi said iiltex" circuit, and ground noise reduction apmiratusV connected to the output of said amplier. x GLENN L. DIMMICK. 

